On the Origin of Species by Means of Natural Selection
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The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with the
Gallinaceae; and this perhaps explains the origin of a singular instinct in
the allied group of ostriches. For several hen ostriches, at least in the
case of the American species, unite and lay first a few eggs in one nest
and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of
eggs; but, as in the case of the cuckoo, at intervals of two or three days.
This instinct, however, of the American ostrich has not as yet been
perfected; for a surprising number of eggs lie strewed over the plains, so
that in one day's hunting I picked up no less than twenty lost and wasted
eggs.
Many bees are parasitic, and always lay their eggs in the nests of bees of
other kinds. This case is more remarkable than that of the cuckoo; for
these bees have not only their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would be necessary if they had to store
food for their own young. Some species, likewise, of Sphegidae (wasp-like
insects) are parasitic on other species; and M. Fabre has lately shown good
reason for believing that although the Tachytes nigra generally makes its
own burrow and stores it with paralysed prey for its own larvae to feed on,
yet that when this insect finds a burrow already made and stored by another
sphex, it takes advantage of the prize, and becomes for the occasion
parasitic. In this case, as with the supposed case of the cuckoo, I can
see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose nest and
stored food are thus feloniously appropriated, be not thus exterminated.
Slave-making instinct. -- This remarkable instinct was first discovered in
the Formica (Polyerges) rufescens by Pierre Huber, a better observer even
than his celebrated father. This ant is absolutely dependent on its
slaves; without their aid, the species would certainly become extinct in a
single year. The males and fertile females do no work. The workers or
sterile females, though most energetic and courageous in capturing slaves,
do no other work. They are incapable of making their own nests, or of
feeding their own larvae. When the old nest is found inconvenient, and
they have to migrate, it is the slaves which determine the migration, and
actually carry their masters in their jaws. So utterly helpless are the
masters, that when Huber shut up thirty of them without a slave, but with
plenty of the food which they like best, and with their larvae and pupae to
stimulate them to work, they did nothing; they could not even feed
themselves, and many perished of hunger. Huber then introduced a single
slave (F. fusca), and she instantly set to work, fed and saved the
survivors; made some cells and tended the larvae, and put all to rights.
What can be more extraordinary than these well-ascertained facts? If we
had not known of any other slave-making ant, it would have been hopeless to
have speculated how so wonderful an instinct could have been perfected.
Formica sanguinea was likewise first discovered by P. Huber to be a
slave-making ant. This species is found in the southern parts of England,
and its habits have been attended to by Mr. F. Smith, of the British
Museum, to whom I am much indebted for information on this and other
subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as any
one may well be excused for doubting the truth of so extraordinary and
odious an instinct as that of making slaves. Hence I will give the
observations which I have myself made, in some little detail. I opened
fourteen nests of F. sanguinea, and found a few slaves in all. Males and
fertile females of the slave-species are found only in their own proper
communities, and have never been observed in the nests of F. sanguinea.
The slaves are black and not above half the size of their red masters, so
that the contrast in their appearance is very great. When the nest is
slightly disturbed, the slaves occasionally come out, and like their
masters are much agitated and defend their nest: when the nest is much
disturbed and the larvae and pupae are exposed, the slaves work
energetically with their masters in carrying them away to a place of
safety. Hence, it is clear, that the slaves feel quite at home. During
the months of June and July, on three successive years, I have watched for
many hours several nests in Surrey and Sussex, and never saw a slave either
leave or enter a nest. As, during these months, the slaves are very few in
number, I thought that they might behave differently when more numerous;
but Mr. Smith informs me that he has watched the nests at various hours
during May, June and August, both in Surrey and Hampshire, and has never
seen the slaves, though present in large numbers in August, either leave or
enter the nest. Hence he considers them as strictly household slaves. The
masters, on the other hand, may be constantly seen bringing in materials
for the nest, and food of all kinds. During the present year, however, in
the month of July, I came across a community with an unusually large stock
of slaves, and I observed a few slaves mingled with their masters leaving
the nest, and marching along the same road to a tall Scotch-fir-tree,
twenty-five yards distant, which they ascended together, probably in search
of aphides or cocci. According to Huber, who had ample opportunities for
observation, in Switzerland the slaves habitually work with their masters
in making the nest, and they alone open and close the doors in the morning
and evening; and, as Huber expressly states, their principal office is to
search for aphides. This difference in the usual habits of the masters and
slaves in the two countries, probably depends merely on the slaves being
captured in greater numbers in Switzerland than in England.
One day I fortunately chanced to witness a migration from one nest to
another, and it was a most interesting spectacle to behold the masters
carefully carrying, as Huber has described, their slaves in their jaws.
Another day my attention was struck by about a score of the slave-makers
haunting the same spot, and evidently not in search of food; they
approached and were vigorously repulsed by an independent community of the
slave species (F. fusca); sometimes as many as three of these ants clinging
to the legs of the slave-making F. sanguinea. The latter ruthlessly killed
their small opponents, and carried their dead bodies as food to their nest,
twenty-nine yards distant; but they were prevented from getting any pupae
to rear as slaves. I then dug up a small parcel of the pupae of F. fusca
from another nest, and put them down on a bare spot near the place of
combat; they were eagerly seized, and carried off by the tyrants, who
perhaps fancied that, after all, they had been victorious in their late
combat.
At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of the nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr. Smith. Although so
small a species, it is very courageous, and I have seen it ferociously
attack other ants. In one instance I found to my surprise an independent
community of F. flava under a stone beneath a nest of the slave-making F.
sanguinea; and when I had accidentally disturbed both nests, the little
ants attacked their big neighbours with surprising courage. Now I was
curious to ascertain whether F. sanguinea could distinguish the pupae of F.
fusca, which they habitually make into slaves, from those of the little and
furious F. flava, which they rarely capture, and it was evident that they
did at once distinguish them: for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much terrified
when they came across the pupae, or even the earth from the nest of F.
flava, and quickly ran away; but in about a quarter of an hour, shortly
after all the little yellow ants had crawled away, they took heart and
carried off the pupae.
One evening I visited another community of F. sanguinea, and found a number
of these ants entering their nest, carrying the dead bodies of F. fusca
(showing that it was not a migration) and numerous pupae. I traced the
returning file burthened with booty, for about forty yards, to a very thick
clump of heath, whence I saw the last individual of F. sanguinea emerge,
carrying a pupa; but I was not able to find the desolated nest in the thick
heath. The nest, however, must have been close at hand, for two or three
individuals of F. fusca were rushing about in the greatest agitation, and
one was perched motionless with its own pupa in its mouth on the top of a
spray of heath over its ravaged home.
Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a
contrast the instinctive habits of F. sanguinea present with those of the
F. rufescens. The latter does not build its own nest, does not determine
its own migrations, does not collect food for itself or its young, and
cannot even feed itself: it is absolutely dependent on its numerous
slaves. Formica sanguinea, on the other hand, possesses much fewer slaves,
and in the early part of the summer extremely few. The masters determine
when and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves seem
to have the exclusive care of the larvae, and the masters alone go on
slave-making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly
the slaves, tend, and milk as it may be called, their aphides; and thus
both collect food for the community. In England the masters alone usually
leave the nest to collect building materials and food for themselves, their
slaves and larvae. So that the masters in this country receive much less
service from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not pretend to
conjecture. But as ants, which are not slave-makers, will, as I have seen,
carry off pupae of other species, if scattered near their nests, it is
possible that pupae originally stored as food might become developed; and
the ants thus unintentionally reared would then follow their proper
instincts, and do what work they could. If their presence proved useful to
the species which had seized them--if it were more advantageous to this
species to capture workers than to procreate them--the habit of collecting
pupae originally for food might by natural selection be strengthened and
rendered permanent for the very different purpose of raising slaves. When
the instinct was once acquired, if carried out to a much less extent even
than in our British F. sanguinea, which, as we have seen, is less aided by
its slaves than the same species in Switzerland, I can see no difficulty in
natural selection increasing and modifying the instinct--always supposing
each modification to be of use to the species--until an ant was formed as
abjectly dependent on its slaves as is the Formica rufescens.
Cell-making instinct of the Hive-Bee. -- I will not here enter on minute
details on this subject, but will merely give an outline of the conclusions
at which I have arrived. He must be a dull man who can examine the
exquisite structure of a comb, so beautifully adapted to its end, without
enthusiastic admiration. We hear from mathematicians that bees have
practically solved a recondite problem, and have made their cells of the
proper shape to hold the greatest possible amount of honey, with the least
possible consumption of precious wax in their construction. It has been
remarked that a skilful workman, with fitting tools and measures, would
find it very difficult to make cells of wax of the true form, though this
is perfectly effected by a crowd of bees working in a dark hive. Grant
whatever instincts you please, and it seems at first quite inconceivable
how they can make all the necessary angles and planes, or even perceive
when they are correctly made. But the difficulty is not nearly so great as
it at first appears: all this beautiful work can be shown, I think, to
follow from a few very simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown that
the form of the cell stands in close relation to the presence of adjoining
cells; and the following view may, perhaps, be considered only as a
modification of this theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of work.
At one end of a short series we have humble-bees, which use their old
cocoons to hold honey, sometimes adding to them short tubes of wax, and
likewise making separate and very irregular rounded cells of wax. At the
other end of the series we have the cells of the hive-bee, placed in a
double layer: each cell, as is well known, is an hexagonal prism, with the
basal edges of its six sides bevelled so as to join on to a pyramid, formed
of three rhombs. These rhombs have certain angles, and the three which
form the pyramidal base of a single cell on one side of the comb, enter
into the composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the
hive-bee and the simplicity of those of the humble-bee, we have the cells
of the Mexican Melipona domestica, carefully described and figured by
Pierre Huber. The Melipona itself is intermediate in structure between the
hive and humble bee, but more nearly related to the latter: it forms a
nearly regular waxen comb of cylindrical cells, in which the young are
hatched, and, in addition, some large cells of wax for holding honey.
These latter cells are nearly spherical and of nearly equal sizes, and are
aggregated into an irregular mass. But the important point to notice, is
that these cells are always made at that degree of nearness to each other,
that they would have intersected or broken into each other, if the spheres
had been completed; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which thus tend to
intersect. Hence each cell consists of an outer spherical portion and of
two, three, or more perfectly flat surfaces, according as the cell adjoins
two, three or more other cells. When one cell comes into contact with
three other cells, which, from the spheres being nearly of the same size,
is very frequently and necessarily the case, the three flat surfaces are
united into a pyramid; and this pyramid, as Huber has remarked, is
manifestly a gross imitation of the three-sided pyramidal basis of the cell
of the hive-bee. As in the cells of the hive-bee, so here, the three plane
surfaces in any one cell necessarily enter into the construction of three
adjoining cells. It is obvious that the Melipona saves wax by this manner
of building; for the flat walls between the adjoining cells are not double,
but are of the same thickness as the outer spherical portions, and yet each
flat portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would probably have been as perfect as the comb of the
hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and this
geometer has kindly read over the following statement, drawn up from his
information, and tells me that it is strictly correct:-
If a number of equal spheres be described with their centres placed in two
parallel layers; with the centre of each sphere at the distance of radius x
sqrt(2) or radius x 1.41421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same layer; and at the same distance
from the centres of the adjoining spheres in the other and parallel layer;
then, if planes of intersection between the several spheres in both layers
be formed, there will result a double layer of hexagonal prisms united
together by pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the same
with the best measurements which have been made of the cells of the
hive-bee.
Hence we may safely conclude that if we could slightly modify the instincts
already possessed by the Melipona, and in themselves not very wonderful,
this bee would make a structure as wonderfully perfect as that of the
hive-bee. We must suppose the Melipona to make her cells truly spherical,
and of equal sizes; and this would not be very surprising, seeing that she
already does so to a certain extent, and seeing what perfectly cylindrical
burrows in wood many insects can make, apparently by turning round on a
fixed point. We must suppose the Melipona to arrange her cells in level
layers, as she already does her cylindrical cells; and we must further
suppose, and this is the greatest difficulty, that she can somehow judge
accurately at what distance to stand from her fellow-labourers when several
are making their spheres; but she is already so far enabled to judge of
distance, that she always describes her spheres so as to intersect largely;
and then she unites the points of intersection by perfectly flat surfaces.
We have further to suppose, but this is no difficulty, that after hexagonal
prisms have been formed by the intersection of adjoining spheres in the
same layer, she can prolong the hexagon to any length requisite to hold the
stock of honey; in the same way as the rude humble-bee adds cylinders of
wax to the circular mouths of her old cocoons. By such modifications of
instincts in themselves not very wonderful,--hardly more wonderful than
those which guide a bird to make its nest,--I believe that the hive-bee has
acquired, through natural selection, her inimitable architectural powers.
But this theory can be tested by experiment. Following the example of Mr.
Tegetmeier, I separated two combs, and put between them a long, thick,
square strip of wax: the bees instantly began to excavate minute circular
pits in it; and as they deepened these little pits, they made them wider
and wider until they were converted into shallow basins, appearing to the
eye perfectly true or parts of a sphere, and of about the diameter of a
cell. It was most interesting to me to observe that wherever several bees
had begun to excavate these basins near together, they had begun their work
at such a distance from each other, that by the time the basins had
acquired the above stated width (i.e. about the width of an ordinary cell),
and were in depth about one sixth of the diameter of the sphere of which
they formed a part, the rims of the basins intersected or broke into each
other. As soon as this occurred, the bees ceased to excavate, and began to
build up flat walls of wax on the lines of intersection between the basins,
so that each hexagonal prism was built upon the festooned edge of a smooth
basin, instead of on the straight edges of a three-sided pyramid as in the
case of ordinary cells.
I then put into the hive, instead of a thick, square piece of wax, a thin
and narrow, knife-edged ridge, coloured with vermilion. The bees instantly
began on both sides to excavate little basins near to each other, in the
same way as before; but the ridge of wax was so thin, that the bottoms of
the basins, if they had been excavated to the same depth as in the former
experiment, would have broken into each other from the opposite sides. The
bees, however, did not suffer this to happen, and they stopped their
excavations in due time; so that the basins, as soon as they had been a
little deepened, came to have flat bottoms; and these flat bottoms, formed
by thin little plates of the vermilion wax having been left ungnawed, were
situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite sides of the
ridge of wax. In parts, only little bits, in other parts, large portions
of a rhombic plate had been left between the opposed basins, but the work,
from the unnatural state of things, had not been neatly performed. The
bees must have worked at very nearly the same rate on the opposite sides of
the ridge of vermilion wax, as they circularly gnawed away and deepened the
basins on both sides, in order to have succeeded in thus leaving flat
plates between the basins, by stopping work along the intermediate planes
or planes of intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of wax,
perceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half-completed rhombs at the base of a
just-commenced cell, which were slightly concave on one side, where I
suppose that the bees had excavated too quickly, and convex on the opposed
side, where the bees had worked less quickly. In one well-marked instance,
I put the comb back into the hive, and allowed the bees to go on working
for a short time, and again examined the cell, and I found that the rhombic
plate had been completed, and had become perfectly flat: it was absolutely
impossible, from the extreme thinness of the little rhombic plate, that
they could have effected this by gnawing away the convex side; and I
suspect that the bees in such cases stand in the opposed cells and push and
bend the ductile and warm wax (which as I have tried is easily done) into
its proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly see that
if the bees were to build for themselves a thin wall of wax, they could
make their cells of the proper shape, by standing at the proper distance
from each other, by excavating at the same rate, and by endeavouring to
make equal spherical hollows, but never allowing the spheres to break into
each other. Now bees, as may be clearly seen by examining the edge of a
growing comb, do make a rough, circumferential wall or rim all round the
comb; and they gnaw into this from the opposite sides, always working
circularly as they deepen each cell. They do not make the whole
three-sided pyramidal base of any one cell at the same time, but only the
one rhombic plate which stands on the extreme growing margin, or the two
plates, as the case may be; and they never complete the upper edges of the
rhombic plates, until the hexagonal walls are commenced. Some of these
statements differ from those made by the justly celebrated elder Huber, but
I am convinced of their accuracy; and if I had space, I could show that
they are conformable with my theory.
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